The present invention relates to a photosensitive CCD device, enabling the analysis of a light signal. It finds a particularly significant application, although not exclusive, in equipment called "lidars" enabling the measurement of the distance from obstacles or from environments which cause back scattering or reflection of short light impulses. Such lidars are particularly used for determining the altitude and the position of clouds from time of flight measurements of back scattered laser impulses by the surface of clouds. A particular type of lidar, called "wind lidar", enables the measurement of the mean speed of the wind in altitude sections, by using the shift of the wave length caused by the Doppler effect.
A photosensitive charge coupled and storage device has already been proposed (FR-A 2735 935) enabling the analysis of a light signal, incorporating a matrix of photo-detectors. This detector, called CCD, shows numerous advantages, above all when a high sensitivity is required. The CCD detectors can be made in a way to have a high quantum efficacy from ultraviolet up to near infrared; they enable bringing together, on a same microchip, charge integration and analogical storage functions. They enable the transfer of charges at a high frequency. Their read-out noise level is low.
The charge coupled device in accordance with patent FR 2735 935 already mentioned includes an image zone constituted by a matrix with N columns of photosensitive sites receiving the light signal, a memory zone constituted by a matrix of N columns of non-photosensitive site columns each line of which receives the charges which appeared in the image zone during an acquisition period (which in the case of the use in a lidar will correspond to a time of the back scattered signal from an impulse laser) and a read-out register of N sites which receives the charges stored in the lines of the memory zone, each in turn. At the border between the image zone and the memory zone the system has an integration zone constituted by a line of N sites in which are accumulated, column by column, the charges collected in the image zone during the acquisition period. The number of lines of the memory zone corresponds to the number of sampling time windows in the case of a lidar. The read-out register is read at the frequency of the laser shots in the case of a lidar, i.e. in intervals corresponding at least to one observation period of the back scattering.
In the case of a wind lidar, the device is connected to a spectrometer enabling the linking of each image zone column with a spectral shift, and therefore to a different speed.
Such a device enables lowering the read-out frequency to a read-out per shot in the case of a lidar. But it requires, for many applications where the intensity of the back scattered light is very weak, a CCD detector having an intensifier element, such as a micro-channel wafer, enabling it to work by counting photons. Indeed, a plain charge coupling device would present a high read-out noise, liable to degrade the measurements in the case of a reader at the close of each observation period, particularly a lidar where the read-out is then at each shot.